1. Field of the Invention
This invention relates to dental impression copings of the type used in implant dentistry to take impressions of a dental implant site from which accurate models can be constructed. More particularly, the invention relates to a universal impression coping system and method for taking accurate dental impressions using either open-tray or closed-tray techniques.
2. Description of the Related Art
Implant dentistry involves the restoration of one or more teeth in a patient's mouth using artificial components, including typically an implant fixture or root and a prosthetic tooth and/or final abutment which is secured to the implant fixture. According to state of the art techniques, the process for restoring a tooth and its root is carried out generally in three stages.
Stage I involves implanting the dental implant fixture into the bone of a patient's jaw. The oral surgeon first accesses the patient's jawbone through the patient's gum tissue and removes any remains of the tooth to be replaced. Next, the specific site in the patient's jaw where the implant will be anchored is widened by drilling and/or reaming to accommodate the width of the dental implant fixture to be implanted. Then, the dental implant fixture is inserted into the hole in the jawbone, typically by screwing, although other techniques are known for introducing the implant in the jawbone.
The implant fixture itself is typically fabricated from pure titanium or a titanium alloy. Such materials are known to produce osseointegration of the fixture with the patient's jawbone. The dental implant fixture also typically includes a hollow threaded bore through at least a portion of its body and extending out through its proximal end which is exposed through the crestal bone for receiving and supporting the final tooth prosthesis and/or various intermediate components or attachments.
After the implant is initially installed in the jawbone a temporary healing cap is secured over the exposed proximal end in order to seal the internal bore. The patient's gums are then sutured over the implant to allow the implant site to heal and to allow desired osseointegration to occur. Complete osseointegration typically takes anywhere from four to ten months.
During stage II, the surgeon reassesses the implant fixture by making an incision through the patient's gum tissues. The healing cap is then removed, exposing the proximal end of the implant. A mold or impression is then taken of the patient's mouth to accurately record the position and orientation of the implant within the mouth. This is used to create a plaster model or analogue of the mouth and/or the implant site and provides the information needed to fabricate the prosthetic replacement tooth and any required intermediate prosthetic components. Stage II is typically completed by attaching to the implant a temporary healing abutment or other transmucosal component to control the healing and growth of the patient's gum tissue around the implant site.
Stage III involves fabricating and placement of a cosmetic tooth prosthesis to the implant fixture. The plaster analogue provides laboratory technicians with a model of the patient's mouth, including the orientation of the implant fixture relative to the surrounding teeth. Based on this model, the technician constructs a final restoration. The final step in the restorative process is replacing the temporary healing abutment with the final restoration.
To achieve optimal results in terms of overall aesthetics and bio-functioning ability of the tooth restoration, it is essential in stage II that the plaster analogue accurately reflect the true position and orientation of the implant in the patient's mouth and that in stage III such position and orientation is faithfully replicated when securing the final tooth restoration to the implant. To help achieve this accuracy and faithful replication, one or more indexing means are typically provided on the proximal end of the implant and corresponding mating indexing means are formed on the various mating components which are adapted to be fitted to the implant. Such indexing means provide desired orientation of the implant and mating components relative to one another and also prevent undesired rotation.
Such indexing means frequently take the form of a hexagonal boss or recess ("hex") formed on the proximal portion of the implant exposed through the crestal bone. For externally threaded implants the hex may also be used to engage a driving tool for driving the implant into an internally threaded bore or osteotomy prepared in the patient's jawbone (mandible or maxilla). When the implant is filly installed in a patient's jawbone the hex or other indexing means is typically exposed through the crestal bone so that accurate indexing may be provided between the implant and the final prosthesis and/or various intermediate mating prosthetic components.
As noted above, during stage II of the dental restorative process a mold or impression is taken of the patient's mouth to accurately record the position(s) and orientation(s) of the indexing means within the mouth at the implant site(s) and to thereby provide the information needed to fabricate the restorative replacement(s) and/or intermediate prosthetic components. According to the state of the art, this is done using a casting or impression material formed of a soft resin--typically polyvinylsiloxane or polyether--which can be applied over the implant site using a suitable impression tray and allowed to cure in situ. The impression material is sufficiently resilient such that it can be removed from the patient's mouth after it is cured (or partially cured) while at the same time retaining an accurate impression of the patient's mouth and particularly the implant site.
However, because the indexing means of the implant is typically quite small and may be recessed partially beneath the gums of a patient, a secondary or intermediate impression element is typically used to help transfer accurately the orientation of the indexing means of the implant. This intermediate impression element is commonly called a "coping" or "impression coping." Examples of impression copings as found in the prior art are shown in U.S. Pat. No. 4,955,811 to Lazzara et al., e.g. FIGS. 5, 6, and 9. There are primarily two types of such impression copings used today--so-called "transfer" impression copings and so-called "pick-up" impression copings.
Conventional transfer impression copings have an impression portion adapted to form a unique or indexed impression in the impression material and a base portion having mating indexing means adapted to mate with the exposed indexing means of the implant. In use, the impression coping is temporarily secured to the exposed proximal end of the implant fixture such that the mating indexing means of the impression coping and implant are interlockingly mated to one another. Typically, a threaded screw or bolt is used to temporarily secure the impression coping to the implant fixture.
Once the impression coping(s) is secured to the implant fixture(s), an impression of the coping(s) relative to the surrounding teeth is taken. A U-shaped tray filled with an impression material is placed in the patient's mouth over the implant site. The patient bites down on the tray, squeezing the impression material into the implant site and around the impression coping(s). Within a few minutes, the impression material cures or hardens to a flexible, resilient consistency. The impression tray is then removed from the patient's mouth to reveal an impression of the implant site and the impression coping(s). The restorative dentist then removes the impression coping(s) from the patient's mouth and transfers the coping(s) back into the impression material, being careful to preserve the proper orientation of the indexing means. This impression method using transfer impression copings is commonly referred to as the "closed-tray" technique.
Conventional pick-up impression copings are similar to transfer copings described above, except that pick-up impression copings typically include an embedment portion adapted to non-removably embed the impression coping securely within the impression material. In this case, once the impression is taken the tray is removed from the patient's mouth. The impression coping(s) remain in the impression material and are "picked up" and pulled away from the patient's mouth along with the impression material. To facilitate such pick-up removal of impression copings, the tray is provided with one or more apertures or openings through which a tool may be inserted to loosen the screw or bolt securing each coping. Thus, this impression technique is commonly referred to as the "open-tray" technique.
The choice of which technique to use (open tray vs. closed tray) is based primarily on individual patient characteristics and the clinician's preference. While the closed-tray technique is somewhat simpler in its design and execution, it is sometimes prone to inaccuracies where sufficient care is not taken during the step of reinserting the impression coping(s) into the impression material. While the open-tray technique is generally more accurate, it is more complex and, as a result, often takes more time to prepare and execute. In terms of patient indications, the open-tray technique is particularly well suited for multi-site dental restoration procedures, especially when the divergence angle between multiple adjacent implants is, for example, greater than 30.degree., or when the dentist wishes to utilize a verification stent to check the accuracy of the working stone model.
Conventionally, open- and closed-tray systems have required different impression copings to be used with each technique. In the closed-tray system, the restorative dentist must be able to remove the tray and the impression material from the transfer impression coping and screw assembly such that the impression coping remains in the patient's mouth after the impression material is removed. As a result, transfer impression copings used for the closed-tray technique tend to be conical or tapered in shape, with the smaller diameter located at the coronal aspect of the coping, and the larger diameter located at the base, or apical aspect, of the coping where the coping connects to the abutment and implant assembly. This conical shape or taper facilitates removal of the impression material while the coping assembly remains in the patient's mouth.
In the open-tray system, the restorative dentist requires the impression copings to remain securely retained within the impression material as the impression tray is removed from the patient's mouth. As a result, pick-up impression copings used for the open-tray technique typically have a protuberant "lip" or similar embedment projection at their coronal aspect, such that the diameter of the lip is larger than the diameter of the immediately adjacent (more apical) area of the transfer coping. This allows for "grabbing" or traction of the impression material as it is being removed from the patient's mouth.
The practice of using different impression copings for open- and closed-tray systems significantly adds to the cost of restorative dentistry. Often, the prosthodontist does not make a final decision whether to use one technique or the other until after the patient is examined on the day of the restorative procedure. At that time a judgement call is made and the prosthodontist may decide to use either the closed-tray technique or open-tray technique depending upon the particular prevailing indications. For example, a particular prosthodontist may generally prefer to use the closed-tray technique, but may choose the open-tray technique for a given patient case because of excessive or irregular angulation of the longitudinal axes of the implants or adjacent teeth. The ability to make such clinical adjustments requires that clinicians have both open- and closed-tray systems on hand. But, maintaining separate inventories of both transfer and pick-up impression copings in the various sizes and types required for clinical use on a range of patients increases the carrying costs, overhead and administrative burdens of the restorative dentist.
Moreover, conventional transfer (closed tray) impression copings are expensive to manufacture and/or suffer from inaccuracies. A key aspect to achieving accurate impressions using the closed-tray technique is ensuring that the transfer impression coping is reinserted into the impression material in the correct orientation. Any inaccuracy in the angular orientation of the impression coping in the impression material will be transferred directly to the plaster analogue. Once the plaster analogue is cast, inaccuracies in the orientation of the impression coping will not be detected until after fabrication of the final restoration and its attempted placement in the patient's mouth.
Most modem transfer copings provide some form of registration means, such as a flat or the like, for helping the prosthodontist or dental technician identify the correct orientation of the transfer coping when reinserting it into the impression material. Nevertheless, even such improved transfer copings are still prone to occasional misorientation during reinsertion when sufficient care is not exercised or when the impression material is particularly soft.
An alternative approach provides a transfer impression coping having multiple registration means symmetrically arranged about a central axis. In this manner, no matter which way the coping is turned or rotated within the negative impression formed in the impression material the symmetry of the design will urge the coping into any one of a number of indexed orientations. See, e.g. U.S. Pat. No. 5,685,715 to Lazzara et. al. While this "self-indexing" coping design ensures a higher probability that an "indexed" orientation will be achieved, it does not necessarily increase the probability that the indexed orientation will faithfully reproduce the orientation of the implant. In particular, the self-indexing feature of these coping designs requires that precise symmetry and tolerances be maintained when machining the coping to ensure that each possible indexed orientation of the coping faithfully reproduces the desired implant orientation. As a result of the different possible indexed orientations, there is an increased risk that even relatively small inaccuracies in the matching of these small parts may create noticeable errors in the orientation of the final restoration. The tighter tolerances and demand for symmetry further increases the expense of manufacturing such self-indexing transfer impression copings.